Return Air Grille Sizes: 3 CFM Charts + Grille Size Calculator

In HVAC, we have to choose the right return air grille size. Everybody knows that an adequately sized grill will ensure smooth airflow (essential). However, not everybody knows what return grill size to choose.

What size return air grille do you need?

There are two ways how to figure out what return grille size you need:

  1. Calculate the return grille size. We will show you how to properly estimate the grille size in CFM as well as in inches. We will use the 500 CFM face velocity and 70% grille free area to make the calculation. To help you out, we created a grille size CFM calculator that makes the calculation automatically (just insert airflow needed in CFM, and you will get a calculated grille size in square inches).
  2. Check the return grille size CFM chart. The grille size chart tells you what grille size you will need for your HVAC system. Below, you will find 3 grille size CFM charts – 1st one for horizontal grilles, 2nd one for square grill, and the 3rd one for vertical grilles.
grille cfm sizes in inches
Big enough return grille sizes will ensure for sufficient airflow to your HVAC system.

What happens if the grill size is not appropriately sized?

  • Choosing a grille that is too small will basically suffocate your HVAC system. The airflow HVAC system needs will not be sufficient if we choose a return grille size that is too small. Small grills can increase the noise levels as well due to increased ‘sucking’ of air.
  • Choosing a grille that is too big is less problematic. The HVAC system will have sufficient airflow but we will unnecessarily have to sacrifice additional space for the grille. This will also increase the overall cost of our HVAC system.

Below the 3 grille size CFM charts, we also solve 3 examples of adequate grille sizing to demonstrate what size grille you will need for your HVAC system.

Note: ‘Grille sizes’ are usually used to determine cold air return vent cover sizes.

Let’s first look at how we can calculate the grille size before we look at the calculator and the all-encompassing grille CFM charts:

How To Calculate Return Grille Size?

Let’s presume that our HVAC system requires 400 CFM airflow. The grille size has to be big enough (but not too big) to allow for 400 CFM airflow.

Now, to properly calculate the return grille size, we have to make these two assumptions:

  1. Face velocity is 500 FPM (feet per minute). This is the speed at which the return grille sucks the air from your house into your ducts.
  2. Free grille area is 70%. As we know, the grille is always bigger than the duct through which the airflow flows. That is because the grille has to be screwed on the wall; for this, it will need to overlook the duct, resulting in some grille free area loss. Some grilles have 50% free area, others 60%, 70%, 80%, or even 90%. The 70% is the standard average we use when accounting for the free grille area.

With these two concepts in mind, we can look at this return grille size formula:

Grille Area (Sq In) = Airflow (CFM) ÷ Face Velocity (FPM) ÷ Free Area (Percentage) × 144

In this equation, we see that we have the desired airflow (measured in CFM), the face velocity (we presume this is a standard 500 FPM), free area (we presume 70% free area), and the 144 factor that converts the grille size into square inches.

By using this equation, we can determine the size of the return grille we need for 400 CFM airflow like this:

Grille Area (400 CFM) = 400 CFM ÷ 500 FPM ÷ 0.7 × 144 = 164 Sq In

That means we will need a grill with at least 164 square inches of area to accommodate for a 400 CFM airflow.

10×10 grille, for example, has a 100 sq in area; it will be too small. 14×20 grille has a 280 sq in area; it will be too big. The closest grille with at least 164 square inches area are 18×10 grill with 180 sq in area and 6×30 grille with 180 sq in area.

This is how we select a grille size in inches based on the required minimum grille size in square inches.

To make this calculation much easier, you can use this calculator:

Return Grille Size Calculator

You just insert the airflow your HVAC system needs (in CFMs), and you will get the minimal area of the grille size you need:

 

Example: Try calculating the minimal grille area for 600 CFM. You just slide the calculator to ‘600’ and you get the calculate grille size ‘247 square inches’. This means you need a grille size with at least 247 sq in area.

How to match the grille area to the actual grille size in inches?

You can use the following return grille CFM chart. There are 3 charts, namely:

  • Horizontal grille size CFM chart. Horizontal grilles have a longer width dimension than the height dimension.
  • Square grille size CFM chart. Square grilles have the same width and height dimensions.
  • Vertical grille size CFM chart. Vertical grilles have a shorter width dimension than the height dimension.

Let’s look at each of these CFM charts in turn:

Horizontal Grille Size CFM Chart

Grille Size (Inches): Grille Area (Square Inches): Airflow (CFM):
4 x 6 Grille 24 Sq In 58 CFM
4 x 12 Grille 48 Sq In 117 CFM
6 x 10 Grille 60 Sq In 146 CFM
6 x 30 Grille 180 Sq In 438 CFM
8 x 16 Grille 128 Sq In 311 CFM
8 x 18 Grille 144 Sq In 350 CFM
8 x 24 Grille 192 Sq In 467 CFM
8 x 30 Grille 240 Sq In 583 CFM
8 x 32 Grille 256 Sq In 622 CFM
8 x 24 Grille 200 Sq In 486 CFM
8 x 30 Grille 220 Sq In 535 CFM
8 x 32 Grille 240 Sq In 583 CFM
10 x 20 Grille 200 Sq In 583 CFM
10 x 24 Grille 240 Sq In 729 CFM
10 x 30 Grille 300 Sq In 525 CFM
12 x 18 Grille 216 Sq In 583 CFM
12 x 20 Grille 240 Sq In 700 CFM
12 x 24 Grille 288 Sq In 875 CFM
14 x 17 Grille 238 Sq In 578 CFM
14 x 20 Grille 280 Sq In 681 CFM
14 x 25 Grille 350 Sq In 851 CFM
14 x 26 Grille 364 Sq In 885 CFM
16 x 18 Grille 288 Sq In 700 CFM
16 x 20 Grille 320 Sq In 778 CFM
16 x 25 Grille 400 Sq In 972 CFM
16 x 26 Grille 416 Sq In 1011 CFM
18 x 24 Grille 432 Sq In 1050 CFM
18 x 26 Grille 468 Sq In 1138 CFM
18 x 30 Grille 540 Sq In 1313 CFM
18 x 32 Grille 576 Sq In 1400 CFM
20 x 24 Grille 480 Sq In 1167 CFM
20 x 25 Grille 500 Sq In 1215 CFM
20 x 30 Grille 600 Sq In 1458 CFM

Example 1: Let’s say we are looking for a horizontal return grille for 500 CFM airflow. If you look at the chart above, you can see that the 10×30 grille allows for 525 CFM airflow. This is the best choice. You could also pick an 8×32 grille will 583 CFM airflow.

Square Grille Size CFM Chart

Grille Size (Inches): Grille Area (Square Inches): Airflow (CFM):
6 x 6 Grille 36 Sq In 88 CFM
8 x 8 Grille 64 Sq In 156 CFM
10 x 10 Grille 100 Sq In 243 CFM
12 x 12 Grille 144 Sq In 350 CFM
14 x 14 Grille 196 Sq In 476 CFM
16 x 16 Grille 256 Sq In 622 CFM
18 x 18 Grille 324 Sq In 788 CFM
20 x 20 Grille 400 Sq In 972 CFM
22 x 22 Grille 484 Sq In 1176 CFM
24 x 24 Grille 576 Sq In 1400 CFM
26 x 26 Grille 676 Sq In 1643 CFM
30 x 30 Grille 900 Sq In 2188 CFM

Example 2: Let’s saw we are looking for a square return grille that can handle 1,000 CFM airflow. The 20×20 grille allows only for 972 CFM airflow; it will be too small. The correct return grille size for 1,000 CFM is 22×22 grille.

Vertical Grille Size CFM Chart

Grille Size (Inches): Grille Area (Square Inches): Airflow (CFM):
12 x 4 Grille 48 Sq In 117 CFM
12 x 6 Grille 72 Sq In 175 CFM
12 x 10 Grille 120 Sq In 292 CFM
14 x 6 Grille 84 Sq In 204 CFM
14 x 8 Grille 112 Sq In 272 CFM
14 x 10 Grille 140 Sq In 340 CFM
16 x 6 Grille 96 Sq In 233 CFM
16 x 8 Grille 128 Sq In 311 CFM
16 x 12 Grille 192 Sq In 467 CFM
18 x 10 Grille 180 Sq In 438 CFM
18 x 12 Grille 216 Sq In 525 CFM
18 x 14 Grille 252 Sq In 613 CFM
20 x 8 Grille 160 Sq In 389 CFM
20 x 10 Grille 200 Sq In 486 CFM
20 x 12 Grille 240 Sq In 583 CFM
20 x 14 Grille 280 Sq In 681 CFM
20 x 16 Grille 320 Sq In 778 CFM
20 x 18 Grille 360 Sq In 875 CFM
22 x 8 Grille 176 Sq In 428 CFM
22 x 10 Grille 96 Sq In 535 CFM
24 x 4 Grille 144 Sq In 233 CFM
24 x 6 Grille 192 Sq In 350 CFM
24 x 8 Grille 288 Sq In 467 CFM
24 x 12 Grille 336 Sq In 700 CFM
24 x 14 Grille 384 Sq In 817 CFM
24 x 16 Grille 432 Sq In 922 CFM
24 x 18 Grille 364 Sq In 1050 CFM
26 x 14 Grille 416 Sq In 885 CFM
26 x 16 Grille 468 Sq In 1011 CFM
26 x 18 Grille 224 Sq In 1138 CFM
28 x 8 Grille 644 Sq In 544 CFM
28 x 23 Grille 180 Sq In 1565 CFM
30 x 6 Grille 240 Sq In 438 CFM
30 x 8 Grille 300 Sq In 583 CFM
30 x 10 Grille 360 Sq In 729 CFM
30 x 12 Grille 420 Sq In 875 CFM
30 x 14 Grille 480 Sq In 1021 CFM
30 x 16 Grille 128 Sq In 1167 CFM
32 x 4 Grille 256 Sq In 311 CFM
32 x 8 Grille 384 Sq In 622 CFM
32 x 20 Grille 640 Sq In 1556 CFM
34 x 8 Grille 272 Sq In 661 CFM
36 x 10 Grille 360 Sq In 875 CFM
36 x 12 Grille 432 Sq In 1050 CFM
36 x 14 Grille 504 Sq In 1225 CFM
38 x 16 Grille 608 Sq In 1478 CFM
40 x 20 Grille 800 Sq In 1944 CFM

Example 3: Let’s say we are looking for a vertical grille size that can handle 800 CFM. Checking the chart above, we see that the 24×14 grille (813 CFM) comes closest (but not under) 800 CFM.

With these charts, you can effectively look up the grille size you need. If you have any questions about these grille sizes or would like for us to calculate your grille size, you can use the comments below and we’ll try to help you out.

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